Add Sentry eventID to response

[Tommatheussen]

Sets the eventId returned from captureException on ExpressJS middleware. This allows us to use the eventId to inform the user of a tracking ID they can use when in contact with Helpdesk.

Implements behaviour as shown in https://docs.sentry.io/platforms/node/express/
Likely an oversight that was missed when moving from Raven to the new clients?

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[kamilogorek]

You are totally right –

sentry-javascript/packages/raven-node/lib/client.js

Lines 582 to 583 in eeab4c1

	var eventId = self.captureException(err, {req: req});
	res.sentry = eventId;

Thanks!